A jif alert is when someone pronounces the file extension of ".gif" with a soft g or "J" sound as in "jiffy" and someone subsequently calls them on it. Technically, "jif" and "gif" are both correct, but only tech-geeks and nerds use the former pronunciation, while 95% of the population uses the latter. Also, the G in .gif stands for "graphics" which has a hard G, therefore .gif shall be pronounced with a hard G unless you just want to be a snob about it.

Guy 1: "Hey guys, did you see these awesome jif's on the chive?"
Guy 2: "Jif alert! It is pronounced Gif, man."
Guy 1: "No it is not, according to the inventors of the format in 1987 who..."
Guy 3: "Don't be a toolbox!"